This invention relates to an improved gas laser discharge tube wherein atoms or molecules of a discharge gas are excited to laser upper levels, and more particularly to a so-called "negative glow" gas laser discharge tube in which an active medium containing laser upper levels is produced in the negative glow part of the discharge.
A negative glow takes place closely along a cathode, and an oblong active medium is desirable. It is therefore desired that the laser discharge electrode have its cathode surface along the optical axis of laser. For this reason all the known negative glow lasers have a long cathode surface along the optical axis of the laser. They are called "transverse discharge" type glow lasers because the direction of discharge is transverse to their optical axis.
Most, not to say all, of the known negative glow lasers are classified as "hollow cathode lasers" having a narrow hollow cylindrical cathode through which the optical axis of the laser is running. One typical hollow cathode laser is described in W. K. Schuebel "Coaxial Hollow Cathode Laser" in IEEE Journal of Quantum Electronics, Vol. QE-6, 1969, pp. 574-575. Another typical hollow cathode laser is shown in Y. Sugawara and Y. Tokiwa "Multiple Anode Hollow Cathode Laser" in the Japanese Journal of Applied Physics, Vol. 9, 1970, pp. 588-589. A multiple anode hollow cathode laser is described by J. A. Piper and C. E. Webb in the Journal of physics D: Applied physics, Vol. 6, 1973, pp. 400-407.
The negative glow part of the discharge, which plays a great role in the negative glow lasers, appears very close to, but at a certain distance from, the cathode surface along the flow of discharge current. The distance is called "cathode dark space length" and is, as well known, determined by the kind of discharge gas, the material of the discharge cathode and the shape of the discharge cathode. The cathode dark space length is usually reduced at a high current density. It is intensively reduced when the discharge cathode is hollow cylindrical so that an ultraviolet radiation on its cathode surface effectively produces electrons discharging from the cathode surface. If the cathode dark space length is reduced, the negative glow part of the discharge will favorably be enclosed in the hollow cylindrical cathode. This phenomenon is called "hollow cathode effect" and explains why a hollow cylindrical cathode is suitable for a gas laser discharge tube.
An improved hollow cathode laser which gives an axial electric field to a discharge tube having a transverse electric field only is disclosed in U.S. Pat. No. 4,031,428. In this improved laser, the hollow consists of a plurality of cathode elements, and the anode consists of a plurality of anode elements. The cathode elements and the anode elements are alternately arranged, so that each cathode element is connected to an anode element which is connected to the next cathode element. An axial electric field is generated between the adjacent cathode and anode electrodes. The axial electric field is effective to distribute heavy atoms uniformly along the optical axis of the laser. Since the cathode is constituted by a plurality of elements, the probability is reduced that discharge becomes an arc discharge.